Linear stepper motor and fixture for the magnetization of the shaft thereof and methods

ABSTRACT

In a preferred embodiment, linear stepper motor, comprising; an annular stator structure; an axially extending, cylindrical, permanent magnet shaft extending coaxially through said annular stator structure; and the axially extending, cylindrical, permanent magnet shaft having a smooth external surface along a portion thereof with axially alternating N and S poles defined circumferentially in an outer periphery of the portion of the axially extending, cylindrical, permanent magnet shaft. The present invention also provides a method of magnetizing the shaft of such a motor and a fixture and method of manufacturing the fixture therefor.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims the benefit of the filing dates ofU.S. Provisional Application No. 60/181,449, filed Feb. 10, 2000, andtitled LINEAR STEPPER MOTOR and No. 60/220,369, filed Jul. 24, 2000, andtitled METHOD AND FIXTURE FOR MANUFACTURE OF A LINEAR STEPPER MOTORSHAFT.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to stepper motors generally and,more particularly, but not by way of limitation, to a novel linearstepper motor, a fixture for the magnetization of the shaft thereof, andmethods of use and manufacture.

[0004] 2. Background Art

[0005] Some linear stepper motors convert rotary motion to linear motionby mechanical means such as through the use of a threaded nut and leadscrew. Conventional linear motors that directly transfer electromagneticenergy in the stator poles to linear movement of a shaft typicallyemploy toothed structures or have relatively complicated slide/statorarrangements. In either case, the manufacture of such motors isrelatively expensive and the motors typically have high parts counts.

[0006] A problem resides in producing a linear motor with a smooth shaftwith alternating N and S poles. One technique is to glue togethercylindrical segments of N and S magnets. That technique, however, istime consuming and results in a somewhat weak structure. Anothertechnique is to roll a cylinder of ferromagnetic material over a flatplate orthogonal to a series of alternating N and S magnetic strips.This technique is somewhat clumsy and suffers from the fact that theresulting magnetized shaft is of fairly weak magnetic strength.

[0007] Some conventional motors are described in the following patentdocuments:

[0008] U.S. Pat. No. 3,867,676, issued Feb. 18, 1975, to Chai et al.,and titled VARIABLE RELUCTANCE LINEAR STEPPER MOTOR, describes such amotor that has toothed structures on the coils and on the linear member.The novelty of the patent appears to reside in the arrangement of thecoils and the manner in which they are energized.

[0009] U.S. Pat. No. 4,198,582, issued Apr. 15, 1980, to Matthias etal., and titled HIGH PERFORMANCE STEPPER MOTOR, describes, in part, avariable reluctance linear stepper motor in which both the stator andthe slider have nonmagnetic materials arranged therein such that fluxleakage is reduced.

[0010] U.S. Pat. No. 4,286,180, issued Aug. 25, 1981, to Langley, andtitled VARIABLE RELUCTANCE STEPPER MOTOR, describes, in part, such amotor having helically toothed stator and slide structures, therespective widths of the teeth having a predetermined relationship.

[0011] U.S. Pat. No. 4,408,138, issued Oct. 4, 1983, to Okamoto, andtitled LINEAR STEPPER MOTOR, describes a linear stepper motor havingtoothed structures on the stator and on the slider. Coil-wound salientpoles are provided on the slider. The novelty of the patent appears toreside in the arrangement of rollers and rails disposed between thestator and the slider

[0012] U.S. Pat. No. 4,607,197, issued Aug. 19, 1986, to Conrad, andtitled LINEAR AND ROTARY ACTUATOR, describes a variable reluctancelinear/rotary motor in which the armature has axial rows of teethradially spaced around the surface thereof. Selective energization ofstator windings provides linear, rotary, or both linear and rotarymotion of the armature.

[0013] U.S. Pat. No. 4,622,609, issued Nov. 11, 1986, to Barton, andtitled READ/WRITE HEAD POSITIONING APPARATUS, describes a variablereluctance positioning device having toothed structures on facingsurfaces of the stator and the armature and with coils placed on thearmature.

[0014] U.S. Pat. No. 4,695,777, issued Sep. 22, 1987, to Asano, andtitled VR TYPE LINEAR STEPPER MOTOR, describes such a motor havingtoothed structures on the stator and on the slider, the toothedstructures on the stator being on coil-wound salient poles. The toothedstructures bear a predetermined relationship therebetween.

[0015] U.S. Pat. No. 4,712,027, issued Dec. 8, 1987, to Karidis, andtitled RADIAL POLE LINEAR RELUCTANCE MOTOR, describes such a motorhaving a smooth double-helix stator shaft and a smooth laminatedarmature of alternate radial pole laminations and spacer laminations.This arrangement permits a balanced flux path and uses the stator andarmature surfaces as slider bearing surfaces.

[0016] U.S. Pat. No. 4,810,914, issued Mar. 7, 1989, to Karidis et al.,and titled LINEAR ACTUATOR WITH MULTIPLE CLOSED LOOP FLUX PATHSESSENTIALLY ORTHOGONAL TO ITS AXIS, describes a variable reluctanceactuator similar in pertinent respects to that described in the '027patent above.

[0017] U.S. Pat. No. 6,016,021, issued Jan. 18, 2000, to Hinds, andtitled LINEAR STEPPER MOTOR, describes a variable reluctance steppermotor similar in pertinent respects to the motor described in the '609patent above. The novelty of the patent appears to reside in the methodof forming the teeth Accordingly, it is a principal object of thepresent invention to provide a permanent magnet shaft for a linearstepper motor that has a smooth, external peripheral surface.

[0018] It is a further object of the invention to provide a linearstepper motor that has low parts counts and is simple and economical tomanufacture.

[0019] It is an additional object of the invention to provide a methodof magnetizing a smooth shaft for a linear stepper motor that is quickand economical.

[0020] It is another object of the invention to provide a fixture formagnetizing a smooth shaft for a linear stepper motor.

[0021] It is yet a further object of the invention to provide a fixturefor magnetizing a smooth shaft for a linear stepper motor that can beeconomically manufactured.

[0022] It is yet an additional object of the invention to provide such astepper motor conventional coils each easily wound around a bobbin.

[0023] It is yet another object of the invention to provide such astepper motor having a shaft that can rotate at any position at anytime, whether or not the motor is on or off or the shaft is movinglinearly.

[0024] Other objects of the present invention, as well as particularfeatures, elements, and advantages thereof, will be elucidated in, or beapparent from, the following description and the accompanying drawingfigures.

SUMMARY OF THE INVENTION

[0025] The present invention achieves the above objects, among others,by providing, in a preferred embodiment, linear stepper motor,comprising; an annular stator structure; an axially extending,cylindrical, permanent magnet shaft extending coaxially through saidannular stator structure; and said axially extending, cylindrical,permanent magnet shaft having a smooth external surface along a portionthereof with axially alternating N and S poles defined circumferentiallyin an outer periphery of said portion of said axially extending,cylindrical, permanent magnet shaft. The present invention also providesa method of magnetizing the shaft of such a motor and a fixture andmethod of manufacturing the fixture therefor.

BRIEF DESCRIPTION OF THE DRAWING

[0026] Understanding of the present invention and the various aspectsthereof will be facilitated by reference to the accompanying drawingfigures, provided for purposes of illustration only and not intended todefine the scope of the invention, on which:

[0027]FIG. 1 is a fragmentary, side elevational view, partially incross-section, of a linear stepper motor constructed according to thepresent invention.

[0028]FIG. 2 is a rear elevational view of the motor of FIG. 1.

[0029]FIG. 3 is an isometric view of a grooved mandrel for use infabricating a fixture for magnetizing the shaft of the motor of FIG. 1.

[0030]FIG. 4 is an isometric view of the mandrel of FIG. 3 with aconductive wire inserted in the grooves of the mandrel.

[0031]FIG. 5 is an schematic, isometric view of the conductive wireshowing the path of a direct current flowing therein.

[0032]FIG. 6 is an isometric view, partially cut-away, showing themandrel of FIG. 3 inserted in a potting fixture.

[0033]FIG. 7 is an isometric view showing a magnetizing fixtureaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] Reference should now be made to the drawing figures on whichsimilar or identical elements are given consistent identifying numeralsthroughout the various figures thereof, and on which parentheticalreferences to figure numbers direct the reader to the view(s) on whichthe element(s) being described is (are) best seen, although theelement(s) may be seen on other figures also.

[0035]FIG. 1 illustrates a linear stepper motor, constructed accordingto the present invention, and generally indicated by the referencenumeral 20. Motor 20 includes a shaft, or slider, 30 having a smoothouter peripheral surface (at least the portion thereof illustrated) andinserted in a stator structure, generally indicated by the referencenumeral 32, for axial back and forth motion of the shaft with respect tothe stator structure.

[0036] Shaft 30 includes a plurality of alternating N and S nonsalientpoles, as at 34 and 36, respectively, formed around the peripherythereof, which poles may be formed as described below. Shaft 30 ispreferably a hollow cylinder of ceramic or rare earth magnetic material,although the shaft may be solid or may have a core of ferromagnetic orother material with a hollow cylinder of the magnetic material disposedaround the core. Shaft 30 can be economically constructed, for example,by conventional extrusion techniques that can produce a shaft of anygiven length or the shaft can be cut to a suitable length from extrudedstock. At least the portion of shaft 30 containing the N and S poles isnon-segmented and is constructed of a single piece of material.

[0037] Stator structure includes first and second, cylindrical, coils 40and 42, respectively, encircling shaft 30, and conventionally wound onfirst and second annular bobbins 44 and 46. Bobbins 44 and 46 are formedof an electrically insulating material such as Delrin®. First and secondbobbins 44 and 46 are spaced apart by a first spacer 50 and the secondbobbin may be spaced apart from an end plate 52 of motor 20 by a secondspacer 54. First and second spacers 50 and 54 may also provide bearingsurfaces for shaft 30, in which case the first and second spacers arepreferably of a material having a high degree of lubricity such asDelrin®.

[0038] First bobbin 44 spaces apart annular pole plates 60 and 62, whilesecond bobbin 46 spaces apart annular pole plates 64 and 66. A steelband 68 surrounds and is in good electrical contact with annular poleplates 60, 62, 64, and 66, thus completing the circular electromagneticcircuit. Annular pole plates 60, 62, 64, and 66 have nonsalient poles Itwill be understood that, by suitable energization of first and secondcoil-wound bobbins in a conventional manner, shaft 30 may be made toincrementally “step” to the left or right on FIG. 1. It will be furtherunderstood that one or both ends of shaft 30 may be attached to, or bearagainst, one or more elements of another device (not shown).

[0039] While motor 20 is shown as having one set of two-phase statorsections, that is, the motor has two coils, it will be understood thatother arrangements are possible as well. For example, two or more setsof two-phase stator sections may be provided for greater power, theadditional sets of stator sections being added serially in a modularmanner.

[0040]FIG. 2 illustrates some of the elements of motor 20 (FIG. 1) andillustrates conductors 70 that are used to energize stator structure 32and mounting holes 72 defined in end plate 52.

[0041] Thus arranged, motor 20 as shown (FIG. 1) in its minimumconfiguration is constructed of only 11 individual elements that may beheld together principally with a suitable adhesive or other conventionalmeans may be provided to secure together the elements of motor 20.

[0042]FIG. 3 illustrates a mandrel 100 that can be used in constructinga fixture for use in magnetizing shaft 30 (FIG. 1). Here a cylindricalmandrel 100 has a plurality of parallel, cylindrical grooves, as at 110,cut in the outer periphery thereof, the groove having a widthapproximating the diameter of a wire conductor to be used in magnetizingshaft 30. Mandrel 100 is constructed of a non-magnetic,non-electrically-conducting material, with the spacing of grooves 110being determined by the final magnetic widths of poles 34 and 36 onshaft 30.

[0043]FIG. 4 illustrates a conductive wire 150 serially disposed ingrooves 110 in mandrel 100.

[0044]FIG. 5 illustrates the current path in conductive wire 150, eachnearly complete circle shown on FIG. 5 representing a turn of conductivewire 150 in one of grooves 110. It will be noticed that the current flowrepresented by the arrows in conductive wire 150 in adjacent turns ofthe conductive wire are in opposite directions.

[0045]FIG. 6 illustrates mandrel 100, with conductive wire 150 placed ingrooves 110, disposed in a cylindrical, hollow potting fixture 200. Inthis step, a suitable potting compound, such as an epoxy material, ispoured into an annulus 210 defined between the outer surface of mandrel100 and the inner surface of potting fixture 200. After hardening, thepotting compound holds conductive wire 150 in place in grooves 110.

[0046]FIG. 7 illustrates a finished magnetizing fixture, generallyindicated by the reference numeral 300. Fixture 300 comprises mandrel100 with an outer coating of potting compound 310 and ends of conductivewire 150 extending therefrom. A central axial bore 320 has been created,or enlarged, through mandrel 100 to bring conductive wire 150 near tothe inner surface of the mandrel or even to be partially exposed, asshown on FIG. 7, if desired.

[0047] Shaft 30 of motor 20 (FIG. 1) can now be inserted into fixture300 and a high level of direct current passed through conductive wire150 to magnetize alternating N and S poles 34 and 36 along a selectedlength thereof. Such an arrangement provides an economical and rapidmethod of magnetizing shaft 30 and nearly any strength of magnetizationcan be provided, depending on the magnet material, since only one quickburst of direct current is necessary and that can be in a wide range ofvoltages.

[0048] Motor 20 (FIG. 1) has a number of important features. Forexample, motor 20 is of a brushless, magnetically coupled,bi-directional, non-arcing design, having long operational life, withpermanently magnetized output shaft 30. Motor 20 runs on conventionalstepper motor drives and can be microstepped for increased resolutionand accuracy. Shaft 30 is the only moving part and it can be rotated360° continuously or intermittently in either direction, at any time andat any linear position, including when motor 20 is not energized. Thereis no conversion of rotary motion to linear motion with the concomitantefficiency losses. There are no lead screws, ball screws, or ballbearings to wear out and no lubrication is required. Motor 20 canoperate in any orientation and is back-driveable (especially at low orzero power input), that is, shaft 30 can be moved by overcoming themagnetic force between the shaft and annular pole plates 64 and 66.Performance of motor 20 can be increased with shorter duty cycles andcan be easily constructed for vacuum environments, that is, it can beconstructed of materials that do not out gas in a vacuum, the lack oflubrication contributing to this feature. Shaft 30 when hollow allowsthe pass-through of electrical, optical, and/or fluid lines, and/or thelike.

[0049] In the embodiments of the present invention described above, itwill be recognized that individual elements and/or features thereof arenot necessarily limited to a particular embodiment but, whereapplicable, are interchangeable and can be used in any selectedembodiment even though such may not be specifically shown.

[0050] Terms such as “upper”, “lower”, “inner”, “outer”, “inwardly”,“outwardly”, “vertical”, “horizontal”, and the like, when used herein,refer to the positions of the respective elements shown on theaccompanying drawing figures and the present invention is notnecessarily limited to such positions.

[0051] It will thus be seen that the objects set forth above, amongthose elucidated in, or made apparent from, the preceding description,are efficiently attained and, since certain changes may be made in theabove construction without departing from the scope of the invention, itis intended that all matter contained in the above description or shownon the accompanying drawing figures shall be interpreted as illustrativeonly and not in a limiting sense.

[0052] It is also to be understood that the following claims areintended to cover all of the generic and specific features of theinvention herein described and all statements of the scope of theinvention which, as a matter of language, might be said to falltherebetween.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A linear stepper motor,comprising; (a) an annular stator structure; (b) an axially extending,cylindrical, permanent magnet shaft extending coaxially through saidannular stator structure; and (c) said axially extending, cylindrical,permanent magnet shaft having a smooth external surface along a portionthereof with axially alternating n and s poles defined circumferentiallyin an outer periphery of said portion of said axially extending,cylindrical, smooth, permanent magnet shaft.
 2. A linear stepper motor,as defined in claim 1 , wherein: said portion of said axially extending,cylindrical, permanent magnet shaft is hollow.
 3. A linear steppermotor, as defined in claim 1 , wherein: said portion of said axiallyextending, cylindrical, permanent magnet shaft has a solid core.
 4. Alinear stepper motor, as defined in claim 3 , wherein: said solid coreis formed from a ferromagnetic material.
 5. A linear stepper motor, asdefined in claim 3 , wherein: said solid core is formed from anon-magnetic material.
 6. A linear stepper motor, as defined in claim 1, wherein: said stator structure includes annular disks of a highlubricity material spacing apart elements of said stator structure andserving as bearing surfaces for said axially extending shaft.
 7. Alinear stepper motor, as defined in claim 1 , wherein: at least saidportion of said axially extending, cylindrical, smooth, permanent magnetshaft is constructed of a single piece of material.
 8. A linear steppermotor, as defined in claim 1 , wherein: said axially extending,cylindrical, smooth, permanent magnet shaft can rotate 360° continuouslyor intermittently in any direction, regardless of whether or not saidlinear stepper motor is energized.
 9. A linear stepper motor, as definedin claim 1 , wherein: said axially extending, cylindrical, smooth,permanent magnet shaft is back-driveable.
 10. A linear stepper motor, asdefined in claim 1 , wherein: said linear stepper motor is constructedto operate in any orientation.
 11. A linear stepper motor, as defined inclaim 1 , wherein: said stator structure has modular stator stacks. 12.A linear stepper motor as defined in claim 1 , wherein: said statorstructure has conventionally wound coils.
 13. A linear stepper motor, asdefined in claim 1 , wherein said linear stepper motor includes nobearings.
 14. A linear stepper motor, as defined in claim 1 , wherein:said linear stepper motor includes no lead screw and no ball screw. 15.A linear stepper motor, as defined in claim 1 , wherein: said linearstepper motor requires no lubrication of any part thereof.
 16. A linearstepper motor, as defined in claim 1 , wherein: said linear steppermotor requires no conversion of rotary motion to linear motion.
 17. Afixture for magnetizing axially alternating N and S poles definedcircumferentially in a portion of an outer periphery of an axiallyextending, cylindrical, smooth shaft, said fixture comprising: (a) ahollow cylindrical mandrel formed from a non-magnetic,non-electrically-conducting material; (b) a conductive wire disposed inparallel, circumferential channels defined in an outer surface of saidmandrel; (c) a potting compound surrounding said mandrel to secure saidconductive wire in place; and (d) a central bore defined axially andcentrally through said mandrel and exposing or nearly exposing saidconductive wire; and (e) said central bore being sized to accept axiallyinserted therein said portion of said axially extending, cylindrical,smooth shaft.
 18. A fixture, as defined in claim 17 , wherein: saidconductive wire is placed in said parallel, circumferential channelssuch that direction of flow in said conductive wire of a direct currentin adjacent ones of said parallel, circumferential channels is inopposite directions.
 19. A method of providing axially alternating N andS poles in a portion of an axially extending, cylindrical, smooth shaftfor a linear stepper motor, comprising: (a) providing a magnetizingfixture comprising: a hollow cylindrical mandrel formed from anon-magnetic material; a conductive wire disposed in parallel,circumferential channels defined in an outer surface of said mandrel; apotting compound surrounding said mandrel to secure said conductive wirein place; and a central bore defined axially and centrally through saidmandrel and exposing or nearly exposing said conductive wire; and saidcentral bore being sized to accept axially inserted therein said portionof said axially extending, cylindrical, smooth shaft; (b) inserting saidportion of said axially extending, cylindrical shaft in said centralbore; and (c) providing a direct current through said conductive wiresaid conductive wire is placed in said parallel, circumferentialchannels such that direction of flow in said conductive wire of a directcurrent in adjacent ones of said parallel, circumferential channels isin opposite directions.
 20. A method, as defined in claim 19 , furthercomprising: providing said conductive wire placed in said parallel,circumferential channels such that direction of flow in said conductivewire of a direct current in adjacent ones of said parallel,circumferential channels is in opposite directions.
 21. A method ofmanufacturing a magnetizing fixture for magnetizing axially alternatingN and S poles defined circumferentially in a portion of an outerperiphery of an axially extending, cylindrical, smooth shaft, saidmethod comprising: (a) providing a plurality of parallel,circumferential channels defined in an outer surface of a cylindricalmandrel formed from a non-magnetic material; (b) placing a conductivewire in said parallel, circumferential channels; (c) providing a pottingcompound surrounding said mandrel to secure said conductive wire inplace; (d) forming a central bore defined axially and centrally throughsaid mandrel and exposing or nearly exposing said conductive wire; and(e) said central bore being sized to accept axially inserted thereinsaid portion of said axially extending, cylindrical, smooth shaft.
 22. Amethod, as defined in claim 21 , further comprising: providing saidconductive wire placed in said parallel, circumferential channels suchthat direction of flow in said conductive wire of a direct current inadjacent ones of said parallel, circumferential channels is in oppositedirections.